1. Field of the Invention
The present invention relates to a protection device for an inverter in which DC power is converted to AC power and supplied to a load and operation is effected with connection to an AC power system.
2. Description of the Related Art
A typical example of a conventional inverter protection device is shown in FIG. 24 and will now be described.
The DC power of a DC power source 1 constituted by a solar battery or a fuel battery, etc. is converted to AC power by an inverter 2 of the PWM (pulse width modulation) controlled type. The AC power has its high-frequency components removed as a result of PWM control by a filter constituted by a reactor 3 and a capacitor 4 and is supplied to a load 9.
The load 9 is also supplied with AC power for general domestic use that is supplied from an AC power system 8 via a circuit breaker 7 and a pole-mounted transformer 6, and operation is effected with the AC power of the inverter 2 interconnected to the AC power system 8. The AC voltage supplied to the load 9 is detected by a voltage detector 10, and a sine wave signal Vs is input into a current reference circuit 12 via a bandpass filter 16. The current reference circuit 12 multiplies the sine wave signal Vs and a control signal Vc output by an amplifier 11 and outputs a current reference I*. The current reference I* and an output current I of the inverter 2 that is detected by a current detector 5 are input into an amplifier 13 and PWM control of the inverter 2 such as to make the current deviation zero is effected via a PWM control section 14 and a drive section 15.
The phase of the current reference I* is approximately coincident with the phase of the AC voltage supplied to the load 9 and high power factor AC power is supplied from the inverter 2.
When a solar battery is used as the DC power source 1, a voltage reference V* is set so as to take out maximum power. The amplifier 11 generates the control signal Vc to the current reference circuit 12 so that an output voltage V of the DC power source 1 is coincident with the voltage reference V*.
But a detailed description of this will be omitted, since it is not directly related to the present invention.
In the above-described distribution system, maintenance and inspection of the load side including the pole-mounted transformer 6 are effected with separation from the AC power system 8 by opening the circuit breaker 7. In this case, the state on the load side is monitored by a voltage relay 17 and a frequency relay 18, and when separation from the AC power system 8 is effected, this is detected by an abnormality detection circuit 19 and operation of the drive section 15 is halted so as to stop operation of the inverter 2.
However, there are situation in which, if the power supplied by the inverter 2 and the power of the load 9 are balanced at the time when separation from the AC power system 8 is effected by opening the circuit breaker 7, the abnormality detection circuit 19 fails to detect an abnormality and the inverter 2 continues functioning. This state is called islanding or reverse charging and is one that is dangerous during maintenance and inspection work.
In particular, if an induction motor is connected as the load 9, there is a tendency for the voltage and frequency of the AC power system 8 to be liable to be maintained by the counter-electromotive voltage of this motor and so it is difficult to detect the islanding.
A frequency fluctuation system, a bandpass filter system, a power fluctuation system and a higher harmonic voltage monitoring system as noted below have been proposed for the prevention of the islanding, and these systems will now be described.
&lt;Frequency fluctuation system&gt;
A phase shift of a fixed low frequency is applied by means of a fluctuation circuit 21 to the phase of the system reference voltage input to the current reference circuit 12. Islanding is detected from the fluctuation of the inverter output frequency that takes place when the circuit breaker 7 is opened. However, with this system, if the inverter power (including reactive power) and the load power are perfectly balanced, the frequency and voltage will be unchanged, with the result is that detection of islanding will not be successful.
&lt;Bandpass filter system&gt;
Because of the characteristic of the bandpass filter 16, the magnitude and phase of the current reference I* of FIG. 24 are as shown in FIG. 25. At times of islanding, if the reactive power supplied by the inverter 2 and the reactive power required by the load 9 are not the same, the frequency of the AC voltage supplied to the load 9 shifts from the rated frequency f.sub.0, the value of the current falls, the effective power balance is lost, the AC voltage falls, the voltage relay 17 detects an abnormality and the inverter 2 is stopped.
The case where the load impedance consists of a resistor R and a reactor L as shown in FIG. 26(a) and has a lagging power factor angle .phi. shown in FIG. 26(b) will now be considered. In a situation in which the inverter 2 is connected to an AC power system with a large power capacity, it operates at the rated frequency f.sub.0 and so, because of the bandpass filter characteristic of FIG. 25, the inverter 2 outputs power with a power factor of 1.
When the circuit breaker 7 is opened, in order that the output power of the inverter 2 balances with the power required by the load 9, the frequency of the output voltage of the inverter 2 rises because of the lagging power factor angle .phi. of the load 9. When it rises to as far as a frequency f.sub.1, the inverter 2 supplies power whose phase lags by the lagging power factor angle .phi., because of the characteristic of the bandpass filter 16. That is, the inverter 2 functions to lower its output frequency. In this case, the value of the current only falls slightly from I.sub.0 to I.sub.1 and the fluctuation of the voltage is not large. This voltage fluctuation is determined by the supplied active power and the active power consumed by the load 9.
However, if the bandpass filter 16 is not present, since the inverter 2 functions in a manner such that there is always a flow of current with a power factor of 1, at the point where a balance is established in the case of FIG. 26(a) the frequency will be infinitely great. (it does not actually become infinitely great, since there is control delay, etc., but it becomes an extremely high frequency).
Thus, the bandpass filter 16 acts to suppress system frequency fluctuation at the time of islanding. In the case of a leading power factor load, the inverter 2 functions to rise its output frequency and the bandpass filter 16 acts to suppress lowering the frequency. Since at the time of islanding, the action of the bandpass filter 16 is directed towards an increase of voltage fluctuation but is directed towards suppression of frequency fluctuation, depending on conditions, detection of islanding may be delayed.
&lt;Power fluctuation system&gt;
The fluctuation circuit 21 in FIG. 24 causes the current reference I* output from the current reference circuit 12 to fluctuate at a low frequency within a set range. And when the circuit breaker 7 is opened, the balance between the power (reactive power and active power) output by the inverter 2 and the load power breaks down and consequently voltage and frequency fluctuation is caused and islanding is detected. However, even with this system, if a large number of inverters 2 are connected in parallel, there are cases in which the phases of the power fluctuation of the various inverters 2 are different from one another and consequently the state is one in which overall there is no power fluctuation and detection is not possible.
&lt;Higher harmonic voltage monitoring system&gt;
Voltage harmonics on the load side are monitored by a higher harmonic detection circuit 20, and when the circuit breaker 7 is opened, islanding is detected through the fact that there is an increase in higher harmonics (3rd, 5th and 7th harmonics). With this system, however, if a large number of loads 9 are used which have capacitor input type rectification circuits as in inverter air conditioning or television equipment, etc., there is a considerable fall in the reliability of detection, since there is an increase in 3rd, 5th and 7th harmonics at normal times.